1. Field of the Invention
The present invention relates to the preparation of water-soluble, high molecular weight organic polymers/copolymers especially adapted as polymeric flocculants for the treatment of waste and other impure waters, and, more especially, to the preparation of high molecular weight, water-soluble acrylic polymers or copolymers essentially devoid of residual monomer or monomers.
2. Description of the Prior Art
It is well known to this art that, for reasons of toxicity, it is exceedingly important to utilize for water treatment, e.g., water purification, organic water-soluble polymers or copolymers containing but very slight amounts of residual monomer or monomers. Indeed, different government regulations have set an acceptable upper limit on the content of residual monomer or monomers (particularly in the case of acrylamide polymers or copolymers) at 0.05% by weight, with respect to the weight of the dry polymer.
The preparation of acrylic polymers or copolymers by photopolymerization under ultraviolet irradiation too is well known to the prior art. Compare U.S. Pat. No. 4,178,221 which features the preparation of water-soluble acrylic polymers of high molecular weight containing little or no free monomer. According to this particular process, an aqueous solution of acrylic monomer or monomers is continuously deposited onto a support in the form of a thin layer, said solution containing a photopolymerization promoter in an amount of 0.005 to 1% by weight with respect to the total weight of the monomer or monomers, and also containing less than 1 mg oxygen per liter of solution; the thin liquid layer is then subjected to irradiation at wavelengths between 300 nm and 450 nm for 1 to 20 minutes, the average power of such radiation being between 20 and 300 watts/m.sup.2, the gaseous atmosphere enveloping the thin liquid layer having an oxygen content of less than 5% by volume, and the support being cooled in order to facilitate heat removal. Next, the thin layer thus subjected to the onset of photopolymerization is maintained on the cooled support and under an atmosphere free of oxygen and is then exposed to a second radiation stage, at wavelengths between 300 and 450 nm for 1 to 20 minutes, the average power of such radiation being between 300 and 2000 watts/m.sup.2 ; thence, in order to effect removal of the residual monomers present in the resulting, at least partially solidified thin layer, the same is stripped from the support and subjected to yet another stage of irradiation, preferably in a cool air atmosphere, said irradiation being at wavelengths of between 300 and 450 nm for 30 minutes to 3 hours, and with the average power of active radiation being between 20 and 300 watts/m.sup.2. The resultant film is subsequently cut into fragments, dried and ground into powder. This process yields polymer having a content of residual monomer or monomers of less than 0.05%. However, it displays the disadvantage of requiring a lengthy period of third-stage irradiation (30 minutes to 3 hours) under conditions differing from those of the earlier irradiation stages. Thus, such prior art process is both time consuming, as well as expensive.
It too has been postulated that the aforesaid lengthy period of irradiation of from 30 minutes to 3 hours could be avoided by simply extracting the residual monomer(s) with any suitable solvent, such as dilute methanol. And indeed the ultimate product of photopolymerization may be treated with methanol to dissolve the residual monomers, but this is a lengthy, costly and difficult operation, especially in light of the hazardous and toxic character of methanol.
Other techniques are also known for reducing the residual monomer content of water-soluble polymers, e.g., those featuring the use of the alkali metal sulfites or metabisulfites. Such sulfites or bisulfites are post-polymerization additives, being introduced after completion of the polymerization into aqueous mixtures containing the polymer, the polymer then being dried. If such compounds are present during polymerization, however, they interfere with the polymerization catalyst and upset the process equilibrium. Utilization of such compounds may thus be considered to be confined to already polymerized products. It is also known that it is fundamentally impossible to attain the desired results by interacting the sulfite or bisulfite with a powder of the polymeric flocculant, such as that obtained after drying. In the case wherein a solid, rubbery product is obtained after polymerization, and still containing a high proportion of residual monomer or monomers (0.5% of acrylamide monomer, for example), no means are provided by the prior art for interreaction between the sulfite or bisulfite and the product of photopolymerization.
Hence, it will be seen that a serious need exists in this art for a facile and speedy process for the preparation of water-soluble acrylic polymers or copolymers having a very low residual content of starting material monomer or monomers, and especially for the preparation of such polymers/copolymers well suited as markedly effective polymeric flocculants.